CSLB C-10 Electrical ContractorEquipment and InstallationEasy
An electrician installs a subpanel in a detached garage fed from the main dwelling panel by a 4-wire feeder (two hots, a neutral, and an equipment grounding conductor). At the subpanel, how must the neutral and equipment grounding conductor be terminated?
- ABoth must be spliced together outside the panel before entering the enclosure
- BThe neutral may be omitted since the EGC alone can carry return current
- COn separate buses, with the neutral bus isolated from the enclosure and not bonded
- DOn the same bus bar, bonded together with a bonding screw
Show answer & explanationAnswer & explanation
Correct answer: C. On separate buses, with the neutral bus isolated from the enclosure and not bonded
Per NEC 250.24(A)(5) and 408.40, a subpanel supplied by a feeder (not a service) must keep the grounded (neutral) conductor isolated from the equipment grounding conductor and the enclosure to avoid parallel neutral current paths. The main bonding jumper is only used at the service or a separately derived system.
Why the other options are wrong
- A. Splicing outside the panel is not a code-recognized bonding method and creates an unlisted connection.
- B. A neutral is required to serve 120V loads; the EGC cannot substitute for it.
- D. This describes service equipment bonding, not a downstream subpanel.
Subpanel Neutral-Ground Isolation
At any panel downstream of the service (or a separately derived system), the grounded conductor must be kept isolated from equipment grounding conductors and the enclosure.
- 4-wire feeder required to a subpanel with separate neutral and ground
- Bonding only occurs at the service or SDS transformer
- Isolated neutral bus prevents parallel current paths on the EGC
Memory trick: Bond once at the source, isolate everywhere downstream.